As an application field of the DC power supply device, which is supplied with AC power as an input, there is a power supply system for a ship or an airplane, for example. This type of a power supply system uses a three-phase generator as a primary power supply, which is supplied with three-phase AC power as an input so as to output a relatively high AC voltage such as AC 200 to 440 Vrms. In order to prevent harmonic ripple current generated when the primary power supply is full-wave rectified from badly affecting the primary power supply or other devices sharing the primary power supply, it is required to provide a DC power supply device capable of performing AC/DC conversion with very low harmonic ripple and high power factor.
In this case, there is known a related art DC power supply device used in the power supply system for a ship or an airplane, in which a three-phase power factor improvement circuit is constructed of a three-phase bridge circuit connected to the three-phase generator via a choke coil and a three-phase power factor control circuit for controlling the three-phase bridge circuit, and a DC/DC converter for stepping down voltage is connected to the latter stage.
The three-phase bridge circuit is a switching circuit in which three sets of series circuits of two switching elements, to which diodes are respectively connected in anti-parallel, are connected in parallel to both ends of an output capacitor, and series connection nodes of the individual series circuits are connected to corresponding phases of the three-phase generator via choke coils, respectively. The three-phase bridge circuit works as a three-phase power factor control circuit utilizing three-phase full-wave rectification by the six diodes, and a step-up circuit including the choke coils of individual phases, the six switching elements, and the six diodes.
The three-phase power factor control circuit detects AC voltage and AC current supplied to the three-phase bridge circuit via the choke coils and performs dq conversion of the AC voltage and the AC current. Then, the three-phase power factor control circuit simultaneously performs calculation for power factor compensation control and current control, and calculation for voltage control based on an output voltage from the three-phase bridge circuit to the DC/DC converter, so as to determine an ON/OFF ratio of each switching element of the three-phase bridge circuit.
Thus, the three-phase bridge circuit directly performs high speed switching of the three-phase AC voltage at a switching frequency of the ON/OFF ratio controlled in synchronization with the three-phase AC voltage. Therefore, control is performed so that current having the same phase as an input voltage waveform flows in a sine wave manner, and thus three-phase full-wave rectification and three-phase power factor improvement utilizing step-up operation are simultaneously performed.
In addition, as to a DC/DC converter for converting a DC voltage into a voltage that can be used by a load device, a DC/DC converter having small size and light weight with small loss is required, and hence a switching DC/DC conversion circuit is common. However, in a power semiconductor element using silicon (hereinafter referred to as Si), reductions of on-resistance and saturation voltage are close to limitations technically, and hence higher efficiency of the DC/DC converter is also in a saturated state. As a result of studying wide-bandgap semiconductors in recent years, a field effect transistor (FET) using gallium nitride (hereinafter referred to as GaN) and a switching device using silicon carbide (hereinafter referred to as SiC), which are capable of high speed switching and have low on-resistance, high withstand voltage, and large current, have started to be used. It is known in general that if an FET using GaN or SiC is used, switching loss can be reduced so that a DC/DC converter having a smaller size and higher efficiency can be provided.
However, most metal-oxide-semiconductor field effect transistors (MOS FETs) made of GaN or SiC, which are being available currently, have a low threshold value of a gate voltage, and hence they may become the on state when the gate voltage is not applied. Therefore, in order to drive the MOS FET made of GaN or SiC, it is necessary to use two power supplies including a positive power supply and a negative power supply, and a driving circuit that applies a positive voltage to the gate of the FET by the positive power supply when turning on the FET, and applies a negative voltage to the gate of the FET by the negative power supply when turning off the FET.
Further, in a case where a MOS FET made of GaN or SiC having possibility to be turned on when the gate voltage is not applied is used in the power supply circuit, the voltage supplied to the DC/DC converter may be short-circuited normally. Therefore, as described in Patent Literature 1, for example, there is a problem in that it is necessary to consider a protection method such as disposing a protection element or a relay for preventing the short circuit.
As described above, in the conventional technique, in a case where a MOS FET made of GaN or SiC, which is capable of high speed switching and has a low on-resistance, a high withstand voltage, and a large current, is used in the power supply circuit, it is necessary to use another power supply for always applying a negative voltage to the gate of the FET even in the state where the power supply circuit is stopped. Therefore, in a full-bridge type DC/DC converter used in a power supply circuit having a large output power, it is necessary to use two sets of power supplies capable of outputting both the positive and negative voltages as the power supply for driving the two FETs connected to the higher voltage. In addition, it is necessary to use at least one set of power supplies capable of outputting both the positive and negative voltages as the power supply for driving the two FETs connected to the lower voltage. As a result, it is necessary to use in total three sets of power supplies capable of outputting both the positive and negative voltages. Therefore, the circuit becomes complicated, and hence it is difficult to downsize the DC power supply device.
Therefore, there is proposed a method described in Patent Literature 2, in which another power supply is used so that a negative gate voltage can be applied to only the FET on the lower voltage side even in the state where the power supply is stopped.